1. Field of the Invention
The present invention relates to a projection display apparatus, specifically to a liquid crystal projector that projects an image modulated through a liquid crystal panel on a screen, and more specifically to a projection display apparatus that projects a three-dimensional (3D) image using reflective liquid crystal devices.
2. Description of the Related Art
With recent digitalization of cinema contents, a digital projector has been attracting more attention as a movie player. Among digital projectors, a reflective liquid crystal projector using reflective liquid crystal devices is spotlighted because it is especially suitable for a cinema because of its capability of providing a smooth high-resolution image.
To realize a high-quality projection image, occurrence of a ghost image in an optical system of the projector should be reduced.
Especially in a case of the reflective type, there is a possibility that light emitted from a lamp is, for example, reflected on a surface of each projection lens, returns to a surface of the reflective liquid crystal device, and is reflected on a screen again forming a ghost image, which degrades the image quality.
Japanese Unexamined Patent Application Publication No. 08-271855 and Japanese Patent No. 3470491 disclose technologies of preventing occurrence of such a ghost image.
Both of the above patent documents disclose a technology of suppressing the occurrence of the ghost image by arranging a quarter-wave plate with an orientation of an optical axis set to either 45 degrees or 135 degrees immediately in front of the projection lens. A mechanism of this technology is explained below.
FIG. 13 shows the basic configuration of a general 3-chip CCD reflective liquid crystal projector.
Light emitted from a lamp LM enters dichroic mirrors 1 and 2 arranged to cross each other via a reflector RE and a polarization converting uniformly illuminating optical system IL.
Two of three primary colors, for example, red (R) and green (G), are directed to a dichroic mirror 3 via a mirror M1, and they are further split into two colors of such as R and G to enter illumination lenses L1 and L2.
The remaining color, such as blue (B), enters an illumination lens L3 via a mirror M2.
Each colored light then passes through a corresponding one of polarization beam splitters PBS1, PBS2, and PBS3 and reflective liquid crystal devices D1, D2, and D3 to be synthesized by a color composition prism PR, enters a projection lens LN (LN1 and LN2), and is projected on a screen S.
Next, a behavior of the light reflected on the surface of each lens of the projection lens LN is shown in FIG. 14.
For simplification, FIG. 14 shows only part of the optical system shown in FIG. 13.
The light from the lamp LM proceeds in the optical system as indicated by the solid arrows.
Because the light reflected by the reflective liquid crystal device D2 passes through the polarization beam splitter PBS2, it enters the projection lens LN via the color composition prism PR in a state of a p-polarized light (an electric field is vibrating in a plane parallel to the paper plane). It is because the polarization beam splitter generally has a nature of transmitting p-polarized light.
The light reflected by the projection lens LN1 (p-polarized light) schematically proceeds as indicated by the dashed arrows, and returns to the surface of the reflective liquid crystal device D2. The light is then reflected on the surface of the reflective liquid crystal device D2 again toward the projection lens LN, and reaches the screen S and appears as the ghost image.
Therefore, to remove the ghost image, a quarter-wave plate WP with the orientation of the optical axis set to either 45 degrees or 135 degrees (zero degree is parallel to the paper plane) was arranged on an output plane of the color composition prism PR as shown in FIG. 15, in the past.
In this manner, the light output from the quarter-wave plate WP is directed to the projection lens LN, for example, in a right-handed circularly polarized state, then left-handed circularly polarized by a mirror reversal when the light is reflected by the projection lens LN, and returns to pass through the quarter-wave plate WP again.
As a result, the light is s-polarized (the electric field is vibrating in a plane perpendicular to the paper plane) in the color composition prism PR, and enters the polarization beam splitter PBS2.
Because the polarization beam splitter PBS2 has a nature of reflecting an s-polarized light, the light is reflected by the polarization beam splitter PBS2, and wasted without returning to the surface of the reflective liquid crystal device D2. In this manner, the ghost image can be removed.